1. Field of the Invention
This invention relates to an IR-ray absorptive compound and an optical recording medium by utilizing the same, particularly to an IR-ray absorptive compound and an optical recording medium which can improve durability in repeated reproduction and light resistance in optical disc or optical card.
2. Related Background Art
Generally speaking, an optical recording medium, for example, an optical disc or optical card, can record information at high density by forming optically detectable small pits of, for example, 1 .mu.m on a recording layer provided on a substrate having a spiral, circular or linear groove thereon.
By scanning of a laser beam converged on the surface of the recording layer, the recording layer absorbs the laser energy to form optical detectable pits, whereby information is written. For example, according to the heat mode recording system, the recording layer absorbs heat energy and can form a concave small pit through evaporation or melting at that site. Here, by use of an organic dye thin film as the recording layer with high reflectance, the optical contrast of the recording pit can be set at a high level. For example, when a polymethine type dye, azulene type dye or pyrilium type dye, etc. with great light absorption relative to a laser beam is used for an organic dye thin film, a light absorptive reflective film exhibiting metallic luster (reflectance 10 to 50%) can be obtained, whereby an optical recording medium capable of laser recording and reflective reading can be obtained. Particularly, when a semiconductor laser with an oscillation wavelength of 700 to 800 nm is used as the laser light source, there is the advantage that the device can be made smaller and lower in cost.
However, organic dye thin films had the problem that recording and reproduction characteristics and storage stability may be lowered, because they generally were changed by heat and light, etc.
To cope with such a problem, U.S. Pat. No. 4,656,121 has already proposed an optical recording medium having an aminium salt or diimonium salt of a triarylamine type compound incorporated in the recording layer containing a polymethine type dye.